-
Extension Bulletin -1901
The New Breed of Solar Devices:
The Freon-Charged Collector December 1985
by C.R. MacCluer Department of Mathematics Michigan State
University
mem. ASHRAE*
Most solar domestic hot water (DHW) systems installed to date
have been hydronic that is, water-filled. A mixture of water and
propylene glycol, a non-toxic antifreeze, is circulated by a small
pump through roof-mounted solar collectors, is heated by incoming
solar radiation, is piped to a hot water storage tank where it
surrenders its heat, and is then returned to the collector to be
reheated. A few DHW systems use silicon or mineral oils for the
heat transfer. More and more, however, Freon is being used as the
transfer material.
Freon is the duPont trade name for the large class of
fluorocarbons known as refrigerants the fluids used in
refrigerators, air-conditioners and heat pumps. All evidence
indicates that refrigerant-charged solar DHW systems are superior
in efficiency, lower in cost, simpler, require less maintenance and
have a longer life span.
How Freon-charged DHW Systems Work
In theory, all refrigerant-charged systems operate similarly.
The collector is filled with liquid Freon. Solar radiation heats
the Freon in the collector and changes it from a liquid to a vapor.
Even though the
incoming solar v radiation boils \ liquid Freon
collector
pump or gravity
Fig. 1 Solar energy boils the liquid Freon. The vapor moves to
the heat exchanger, where the Freon condenses, passing the latent
heat to the storage water.
Freon temperature does not change, enormous amounts of heat are
stored as latent heat by this change. The Freon vapor then moves by
its own vapor pressure to a heat exchanger, where it condenses,
yielding the latent heat of vaporization to the storage water. The
condensed Freon liquid is then returned by pump or gravity to the
collector so the cycle can begin again (Fig. 1).
In practice, there are two basic types of refrigerant-charged
DHW systems: active and passive, or thermosiphoning systems. An
active system requires a small pump to return the Freon to the
collector; a passive system does not.
Fig. 2 shows a typical active system built around the Solaraide9
tank with integral heat exchanger recendy introduced by Rheem
Manufacturing. Condensed
Cooperative Extension Service Michigan State University
-
FREON LINES
existing 50-gallon gas water heater
check valve
/ RHEEM Solaraide
reservoir with float switches
motor
magnetically driven filter-drier pump
Fig. 2 An active system charged with Freon-114.
Freon-114 leaves the Solaraide heat exchanger and enters a small
reservoir, where the liquid level is sensed magnetically. The pump
turns on when the reservoir is full, off when empty. Electronic
decision and control devices are not required.
The simplest of all solar DHW systems is the true thermosiphon
(Fig. 3). In a thermosiphon, the condensate falls back to the
collector by gravity. This means that the collector must be located
below the storage tank. Sometimes the storage tank is located
within the same enclosure as the collector that is, mounted on the
roof. Proper tank insulation and the diode action of the
thermosiphon prevent the storage water from freezing. The supply
and return water lines must be protected against freezing with heat
tapes or by some other means.
The hybrid DHW system appears in many variations. In Fig. 4, the
heat exchanger is located above the collector and the condensed
Freon returns to the collector by gravity. However, the storage
tank is located below the exchanger, so the storage water or some
second transfer fluid must be pumped through the exchanger. Another
common variation has the heat exchanger located within the
collector itself the water pump runs at a trickle during
non-collection to prevent freeze-up. The disadvantage of such
systems is their complexity.
TABLE I. Manufacturers of Freon-Charged Solar DHW Systems
Company Code Address Colt Energy Sys. Dell Solar Eagle Comm. Serv.
Energy Engr. Golden Horizons Norsun SOESI Solar Research Sundial
Sunland Industries Suntime Urja Western Solar
P H H
A,P H H H H A H P P H
1720 Commerce Way, Paso Robles, CA 93446 1 Second St., New
Rochelle, NY 10801 P.O. Box 1100, Cottonwood, AZ 86326 4616 McLeod
NE, Albuquerque, NM 87109 P.O. Box 1315, Ronkonkoma, NY 11779 P.O.
Box 760, Manotick, Ont K0A 2N0 10639 S.W. 185th St., Miami, FL
33157 525 N. Fifth St., Brighton, Ml 48116 RD 2, Box 352, Avondale,
PA 19311 4024 E. Broadway, Phoenix, AZ 85040 1 Chase St., Bridgton,
ME 04009 3401 W. Fordham Ave., Santa Ana, CA 92704 2506 Zurich Dr.,
Fort Collins, CO 80524
The above list was compiled from magazines Solar Age and Solar
Engineering and Contracting.
-
Advantages of Freon-charged Systems A refrigerant-charged
collector is
inherendy more efficient. A typical hydronic collector
experiences a rise of 30 degrees F from inlet to outlet. Because
Freon traps energy by changing states, not by increasing
temperature, losses from enclosure and piping are lower.
Second, there is no more efficient process than condensation for
heat exchange. Even a slight difference in temperature will produce
condensation. In short, the exchange of heat to the storage water
is very efficient.
Third, hydronic systems require electronic decision and control
devices to turn on and shut off the pump, and they respond
sluggishly to changing solar conditions. Worse yet, their
temperature sensors drift off calibration. Freon systems respond
instantiy and accurately by "boiling." Therefore, more of the
available solar energy is utilized.
Fourth, Freon does not degrade. The antifreeze-water mixtures
used in hydronic systems break down over time and must be replaced
periodically. Hydronic systems are also subject to scaling,
corrosion, erosion, outgassing and galvanic action. Like a
refrigerator, a properly designed Freon system should last 20 years
without maintenance.
Choosing a System Table I lists manufacturers of
refrigerant-charged solar DHW systems, coded A, P or H to denote
active, passive or hybrid systems. Contact these firms for detailed
information on their products and nearby installers. The required
collector area will be smaller than that of comparable hydronic
systems. Passive systems are, of course, less expensive because of
their simplicity. Pay special attention to freeze protection with
hybrid
to existing hot water tank
Rheem Solaraide storage tank
yard-mounted collector
Fig. 3 A passive system charged with Freon-114.
-
pump
Fig. 4 A hybrid system: condensate falls back to the collector
by gravity heat is transported to storage by a second loop.
types. In the past, hybrid systems have been lost because of
water pump failure in one case, because of a blown fuse. Most of
all, look for simplicity, the cardinal rule of good
engineering.
Safety Freon DHW systems use either
Freon-114 (duPonfs recommendation) or Freon-n. The reason is
twofold. Freon-114 and Freon-n are low pressure refrigerants that
enable the use of standard hydronic collectors, and these two
Freons are two of the most benign of all refrigerants. Humans can
tolerate R-114 concentrations as high as 20 percent by volume for
as long as 2 hours without apparent harm. Freon systems are not
do-it-yourself projects, however. Installation requires the special
skills, tools and safety habits of a refrigeration technician.
Homeowners eager to reduce costs might contract with a
refrigeration technician and agree to supply construction labor
while the technician supplies the special refrigeration skills.
One-of-a-kind systems should be designed by a member of ASHRAE* and
built by a qualified refrigeration technician.
* ASHRAE: The American Society of Heating, Refrigerating and
Air-Conditioning Engineers.
H 0-16021
MSU is an Affirmative Action/Equal Opportunity Institution.
Cooperative Extension Service programs are open to all without
regard to race, color, national origin, sex, or handicap. Issued in
furtherance of Cooperative Extension work in agriculture and home
economics, acts of May 8, and June 30, 1914, in cooperation with
the U.S. Department of Agriculture. W.J. Moline, Director,
Cooperative Extension Service, Michigan State University, E.
Lansing, Ml 48824.
This information is for educational purposes only. Reference to
commercial products or trade names does not imply endorsement by
the Cooperative Extension Service or bias against those not
mentioned. This bulletin becomes public property upon publication
and may be reprinted verbatim as a separate or within another
publication with credit to MSU. Reprinting cannot be used to
endorse or advertise a commercial product or company.
1P-2M-12:85-TCM-UP New, Price 45 cents. FILE 18.1